The invention relates generally to hydrostatically supported rolls and, more particularly, to an improved damping device for such a roll.
A roll with a rotatable hollow cylinder, which forms the working roll circumference, and a stationary crosshead penetrating the hollow cylinder lengthwise and leaving clearance all around to the inner circumference of the hollow cylinder, whereby the hollow cylinder is radially supported in an active plane on this crosshead by means of a supporting device and the hollow cylinder is radially movable as a whole opposite the crosshead and can be braked before contact is made, is disclosed in German patent No. 30 04 916, as the bottom roll of a calender. In the illustrated embodiment, the supporting device includes several piston-like supporting elements, which are arranged in rows along the crosshead and are movable in radial cylinder bores of the crosshead. The supporting elements abut against the inner circumference of the hollow cylinder with a hydrostatic bearing surface, in the direction of the roll nip. At its extremities, the hollow cylinder is supported in a guide ring, which can slide on the crosshead in a straight guide, parallel to the active plane of the roll. Depending on how far the piston-like supporting elements are pushed out of the cylinders in the crosshead, the hollow cylinder lifts itself as a whole opposite the crosshead, or it drops when the supporting elements descend.
The hollow cylinders of the type in question, which are used in paper industry applications, can have diameters of up to 1 m and lengths of up to 10 m. They can weigh up to 20 T. Therefore, components of substantial weight are concerned, which, in certain situations, can shift with considerable speed radially opposite the crosshead. The deformation energy stored in the deflected crosshead is also significant. When the load is suddenly removed, this deformation energy can cause the crosshead to spring back within the hollow cylinder. At this end of the displacement path, the brackeless shifting can cause heavy impacts to take place when the inner circumference of the hollow cylinder butts against the cross head. This can be harmful to the roll and the load bearing construction. In particular, however, the inner peripheral area of the hollow cylinder can be damaged. This area is sued primarily as the contact surface for seals and can no longer fulfill this function when dents appear.
An especially fast displacement occurs, if a roll of the type in question, as is the case in German Patent No. 30 04 916, is the bottom roll of a calender featuring quick ventilation. If, in this connection, the paper web tears or another malfunction occurs, the calendar must be quickly opened. This means that the rolls must be separated from each other by an amount of several millimeters. This occurs in German Patent No. 30 04 916, in which the hollow cylinder of the bottom roll is lowered very quickly opposite the cross head by an amount corresponding to the sum of the nips. The liquid in the cylinders of the individual supporting elements is drawn off through a large flow cross section. However, before an end stop is reached, this hydraulic fluid flowing out of the individual cylinders is increasingly throttled, so that a smooth braking of the dropping movement is assured.
The effectiveness of this braking action is tied to the function of the throttle device in the control system of the calender If this throttle device breaks down, or if somewhere a pipe in the liquid guideway bursts or becomes leaky, the hollow cylinder will drop with full speed toward the crosshead and can crash into it violently. A single occurrence of this type can ruin the hollow cylinder.
Thus, there is a need to effect a more reliable damping of the radial movement of the hollow cylinder towards the crosshead.